Introduction
The small intestine, the primary gastric organ in the human body, is a part of the digestive system. Most of the digestive activities occur in this organ. So, what is the small intestine can be answered as – it is a tubular element within the abdominal cavity which carries food from stomach to colon. The intestine includes the small intestine, the large intestine, and the rectum. Many of the main functions, including most of the absorption of water and nutrients, happen in the intestines. The small intestines are the location where 90% of the digestion and absorption actions occur, whereas the other 10% occur in the stomach and large intestines.
Structure and Components
The small intestine, also known as the small bowel, is a long, continuous tube that runs from the stomach to the anus. It is about 20 feet long and almost an inch in diameter. Absorption is helpful in the small intestine as it contains a large surface area on the inside. The task of the small intestines is to break down and disintegrate food from the stomach and absorb almost all of the nutrients from the food.
There are three parts in order of the small intestines. The first part is named as the duodenum, the middle part is called the jejunum, and the last part is known as the ileum. The duodenum’s purpose is to complete the first phase of digestion.
Roles of the Small Intestines
Most of the digestion and absorption of food occurs in the small intestine, and the remaining takes place in stomach and large intestine.
The small intestines help in churning and mixing the consumed food, creating it into chyme, and moving it all along the length till the colon. It combines the ingested food with the mucus to make it easier to move and receive the digesting enzymes from the pancreas and the liver through the pancreatic and common ducts of bile. It breaks down food and water with the digestive enzymes into a thinner form which can be absorbed and then incorporate the nutrients to the tissues of the body, which in return get stronger. All the chemical digestion takes place in the small intestines, therefore making it one of the most significant parts of the digestive system.
What Happens to Food in the Small Intestine?
The following account describes the process of digestion in the small intestine.
The digestion of proteins and carbohydrates starts in the stomach and completes in the small intestine with the help of pancreatic and intestinal juice. However, lipids reach the intestine undigested.
The combination of pancreatic juice and intestinal juice creates a liquid medium that helps in absorption. The digestive enzymes are secreted from the plasma membrane of microvilli.
Chemical digestion requires chyme and its mixing with pancreatic juice and bile. It helps in an upward adjustment of the low pH of chyme.
Functions of the Small Intestines
Since the small intestines have a lot of significance, it also has a lot of functions it needs to carry.
Digestion is Majorly Done by the Small Intestines.
The duodenum absorbs the half-digested food, along with other digestive juices from the liver, pancreas, and its walls. Bile juices that convert fat into easily absorbable liquid are secreted by the liver. The pancreas creates pancreatic juice that helps break down fats into fatty acids and glycerol. Furthermore, the walls of small intestines produce intestinal juice which breaks down starch and carbohydrates into a simple form of sugar, which is also known as glucose, and proteins into amino acids.
Small intestines absorb the digested food.
The blood vessels absorb the digested food into the intestines. This material is then carried to different parts of the body, providing strength. Glucose is broken down to transform oxygen and carbon dioxide, which are crucial components in respiration.
Each component of the small intestines has different functions it needs to attend to. For example:
The duodenum absorbs iron (Fe).
The jejunum absorbs most of the nutrients.
The terminal or ending part of the ileum absorbs vitamin B12 and bile salts.
After all this, material that has not been broken down or disintegrated in the small intestines then passes on to the large intestine, starting another process all over again.
FAQs on Small Intestine
1. What is the small intestine and what are its main parts?
The small intestine is a long, coiled tube that is a crucial part of the digestive system, located between the stomach and the large intestine. It is the primary site for the chemical digestion of food and the absorption of nutrients. It is divided into three distinct sections: the duodenum, the jejunum, and the ileum.
2. What are the two primary functions of the small intestine?
The two primary functions of the small intestine are:
- Chemical Digestion: It breaks down complex food molecules like carbohydrates, proteins, and fats into smaller, absorbable units using enzymes from the pancreas and the intestinal wall, along with bile from the liver.
- Nutrient Absorption: It absorbs the vast majority of nutrients from digested food—including sugars, amino acids, fatty acids, vitamins, minerals, and water—into the bloodstream.
3. How is the structure of the small intestine adapted for maximum absorption?
The small intestine's structure is highly specialised to maximise the surface area for nutrient absorption. Its key adaptations are:
- Extreme Length: It is approximately 6-7 metres (about 20-22 feet) long, providing a long transit time for food.
- Circular Folds (Plicae Circulares): These are large, permanent folds in the intestinal lining that slow the passage of food and increase surface area.
- Villi and Microvilli: The lining is covered in tiny, finger-like projections called villi. Each cell on the surface of the villi has even smaller projections called microvilli. Together, they create a massive surface area, often compared to that of a tennis court, for efficient absorption.
4. How do the duodenum, jejunum, and ileum differ in their specific roles?
While all parts of the small intestine are involved in digestion and absorption, they have specialised roles:
- Duodenum: This is the first and shortest section. Its main role is intensive chemical digestion. It receives acidic chyme from the stomach and neutralises it with alkaline secretions. It also receives bile from the liver and digestive enzymes from the pancreas.
- Jejunum: This is the middle section. Its lining has a very large surface area with long villi, making it the primary site for the absorption of carbohydrates and proteins.
- Ileum: This is the final and longest section. It is responsible for absorbing specific nutrients not absorbed earlier, such as vitamin B12 and bile salts. It also contains immune tissue known as Peyer's patches.
5. What role do villi and microvilli play in digestion?
Villi and microvilli are essential for the final stages of digestion and absorption. Villi are the larger, finger-like folds of the intestinal wall that increase the surface area. The surface of these villi is covered in microvilli, which form the 'brush border'. This brush border contains critical enzymes (like disaccharidases and peptidases) that complete the breakdown of carbohydrates and proteins into their simplest forms, which can then be absorbed directly across the cell membrane into the bloodstream.
6. Which major digestive juices and enzymes are active in the small intestine?
The small intestine uses a combination of secretions to digest food:
- Bile: Produced by the liver and stored in the gallbladder, bile emulsifies fats, breaking large fat globules into smaller droplets for enzymes to act upon.
- Pancreatic Juice: Secreted by the pancreas, it contains enzymes like pancreatic amylase (for carbs), trypsin (for proteins), and lipase (for fats). It also contains bicarbonate to neutralise stomach acid.
- Intestinal Juice (Succus Entericus): Secreted by the intestinal walls, it contains enzymes like maltase, lactase, sucrase, and peptidases that complete the digestive process.
7. Why does the environment in the duodenum become alkaline after receiving acidic chyme from the stomach?
The environment in the duodenum must become alkaline for two key reasons: to protect the intestinal lining from acid damage and to activate digestive enzymes that only function in an alkaline pH. This is achieved by secretions of bicarbonate from two sources: the pancreas (in pancreatic juice) and the Brunner's glands located in the submucosa of the duodenum itself. This neutralises the highly acidic chyme, creating the optimal conditions for digestion.
8. How does the small intestine contribute to the body's immune system?
Beyond digestion, the small intestine plays a vital role in immunity. It contains a significant amount of Gut-Associated Lymphoid Tissue (GALT). The most prominent examples are the Peyer's patches, which are concentrated in the ileum. These patches are clusters of lymphatic cells that monitor the intestinal contents for harmful pathogens, preventing them from entering the bloodstream and triggering an immune response when necessary.
9. How is the final digestion of carbohydrates, proteins, and fats completed in the small intestine?
The final breakdown of macromolecules occurs in the small intestine:
- Carbohydrates: Polysaccharides are broken down by pancreatic amylase, and the resulting disaccharides are broken into monosaccharides (like glucose) by brush border enzymes such as maltase, sucrase, and lactase.
- Proteins: Large proteins are broken into smaller peptides by enzymes like trypsin from the pancreas. These peptides are then broken down into individual amino acids by peptidases on the brush border.
- Fats: Fats are first emulsified by bile into smaller droplets. Pancreatic lipase then breaks these droplets down into fatty acids and glycerol, which can be absorbed.
